The present invention pertains to a variable resistance device utilizing a variable viscosity material for use as a brake and/or clutch. The variable viscosity material may be an electro-rheological fluid, magneto-rheological fluid, ferrofluid, or magnetizable powder. The present invention also pertains to a variable force dampener device utilizing the variable viscosity material for use in dampening an applied force. More particularly, the present invention pertains to a variable resistance device for use in driving and/or braking of a vehicle, wind-to-energy conversion, a reel for controlled unwinding of a line, a variable resistance exercise device, a safety brake for a circular saw, or other applications where smooth rotational braking and coupling is desired. The invention further pertains to a variable force dampener for use in a vehicle suspension, earthquake proofing of buildings, engine mounts, dampening vibration of a helicopter rotor or airplane propeller, a high-precision weapons mount, or other applications where dynamic force dampening is desired. The present invention also pertains to an electric powered vehicle having a drive train and braking system utilizing the inventive variable resistance device, and a suspension system utilizing the inventive variable force dampener.
A conventional braking system requires the direct contact of two friction surfaces, so that the kinetic energy of the object being braked is dissipated through the direct contact of the friction surfaces. Examples of a conventional braking system can be found in automotive disk brakes and drum brakes.
A conventional clutch, on the other hand, is designed to couple a first driven rotational member with a second rotational member, so that through the coupling of the clutch, the second rotational member is also driven. Again, two friction surfaces are brought into contact, under gradually increasing contact pressure, until a desired coupling of the rotational members occurs. An example of a conventional clutch can be found in the drive train of an automobile.
The required direct contact between friction surfaces makes a conventional brake and a conventional clutch subject to degradation due to wearing. Also, conventional brakes and clutches require complicated and numerous parts, making them expensive to manufacture, difficult to maintain and prone to failure.
Alternative clutches and brakes have been developed that utilize a variable viscosity material. In these devices, a rotating member is confined within a hollow member so as to define a gap therebetween. The gap is filled with a variable viscosity material. Upon application of an applied field, the variable viscosity material undergoes a change in flow characteristics effective to variably couple the rotating member to the hollow member, without ever requiring direct frictional contact between the rotating member and the hollow member. In the case of a brake, the hollow member is fixed, so that as the viscosity of the variable viscosity material increases in response to an applied field, a variable braking force is applied to slow or stop the rotation of the rotating member. In the case of a clutch, the rotating member (driver member) is rotated by, for example, a motor. The hollow member is rotationally driven by a variable coupling with the rotating member, as the viscosity of the variable viscosity material increases in response to an applied field. These devices have the tremendous advantage of no wearing surfaces and few parts, making them relatively uncomplicated and easy to maintain as compared with the conventional clutches and brakes that require friction contact between surfaces.
One type of variable viscosity material is known as an electro-rheological fluid (ER fluid). ER fluids exhibit changes in rheological behavior in response to an applied electrical field. The properties of ER fluids have been studied since their initial observations in the 1940's by Winslow (see, Winslow, W. M., 1949 "Induced Fibrillation of Suspensions," Journal of Applied Physics, 20(12): 1137-1140). It was observed that a finely dispersed suspension of starch or silica gel in mineral oil exhibits an increase in flow resistance when exposed to electrical fields on the order of 1 kV/mm (W. Winslow, U.S. Pat. No. 2,417,850, Mar. 25, 1947). An ER fluid comprises, generally, fine particles (usually 1 to 100 mm in diameter) dispersed in a carrier fluid. In some cases, a surfactant is added to help suspend the particles in the fluid.
The electro-rheological phenomenon will now be described with reference to FIGS. 54(a) through 54(c). The electro-rheological effect is due to an interaction between charges placed on electrodes and those in the particles dispersed in the ER fluid. When no charge is on the electrodes (FIG. 54(a), the ER fluid device is electrically neutral. Charges, which can be either positive (ions, protons) or negative (ions, electrons), are free to move in the ER fluid. When voltage is applied (FIG. 54(b), a positive charge is acquired by one electrode while a negative charge is acquired by the other electrode, thereby, applying an electrical field to the ER fluid disposed between the electrodes. Charges in the particles dispersed in the ER fluid react by shifting the negative charge to the particle side nearest the positive electrode, and the positive charge to the particle side nearest the negative electrode. After the charges re-orient, the particles react to the local electrical field by lining up with their positive and negative ends touching in a chain-like formation. The re-orientation and particle alignment occur nearly simultaneously, in milli-seconds. When the chains of particles are subjected to a shearing force (FIG. 54(c)), the charges still attract, even though the particles are pulled away from each other. This attraction is the basis of the ER effect and is experienced as shear resistance. The amount of applied voltage on the electrodes determines the amount of charge that moves in the particles, and is thus directly proportional to shear resistance. When the chains are physically pulled beyond the strength of their attractive force, they brake, reform, and brake again. Yield strength represents the condition when the reforming and braking cycle reaches equilibrium.
The electro-rheological effect has been defined as an apparent change in the viscosity. From a macroscopic point of view, a change in apparent viscosity does occur. However, the actual plastic viscosity of an ER fluid, defined as the change in stress per unit change in shear strain rate, remains approximately constant as the applied electrical field is varied. The parameter that changes is the amount of shear stress needed to initiate flow. For this description, the ER effect will be considered a change in viscosity of the ER fluid in response to an applied field. It is generally accepted that the ER phenomenon originates from particle polarization induced by an electrical field. The observed changes in mechanical properties exhibited by an ER fluid are a direct result of the formation and breakdown of the induced particle-chain network.
FIGS. 53(a) and 53(b) show an ER fluid clutch as described in the prior art literature (see, P. Coulter, et al. "Electro-Rheological Materials and Their Usage in Intelligent Material Systems and Structures," Technomic Publishing Company, Inc., 1992) FIG. 53(a) shows a concentric cylinder clutch configuration in which an inner rotating cylinder is disposed within an outer sealed cylinder so as to define a gap therebetween. An ER fluid is disposed in the gap, and a potential is applied to the inner rotating cylinder and the outer sealed cylinder. With no electrical field present, rotation of the inner rotating cylinder creates a shear stress between the inner rotating cylinder and the outer sealed cylinder, but little or no torque is transmitted to the outer sealed cylinder. In this case, the ER fluid has low viscosity, and thus there is little or no coupling of the rotation of the inner rotating cylinder to the outer sealed cylinder. When an electrical field is applied, the ER fluid stiffens in proportion to the field strength, and stress is transferred to the outer sealed cylinder as torque. If the outer sealed cylinder is free to rotate, the mechanism acts as a clutch with the ER fluid providing a variable coupling mechanism between rotation of the inner rotating cylinder and the outer sealed cylinder. If the outer sealed cylinder is fixed, the mechanism acts as an ER fluid brake, whereby the variable viscosity of the ER fluid provides a variable braking force on rotation of the inner rotating cylinder.
Since the ability of the ER phenomenon to act as a coupling mechanism greatly depends on the strength of the applied electrical field, the electrode surface area available for applying the electrical field is very important. In the configuration shown in FIG. 53(a), the electrode surface area is limited to the peripheral surface area of the inner rotating cylinder. A more advanced ER clutch configuration, shown in FIG. 53(b), is known as a parallel disk clutch configuration. In this configuration, the available electrode surface area is increased by providing the inner rotating member as a series of disks fixed to a rotating shaft. The sealed outer cylinder has a plurality of electrode surfaces each disposed between the respective disks of the rotating inner member, so that potential applied to the disks of the inner rotating member and the electrode surfaces of the outer sealed cylinder applies an electrical field to the ER fluid disposed therebetween. This configuration offers an increase in electrode surface area, and thus the electrical field is applied to a greater volume of ER fluid as compared with the electrical field applied by the electrode surface area available from the ER fluid clutch configuration shown in FIG. 53(a).
The electrical field applied to the ER fluid disposed between the electrode surfaces is proportional to the gap distance between the electrode surfaces. Therefore, to increase the applied electrical field, and thus greatly enhance the ER fluid effect, the gap distance must be minimized. It is extremely difficult to provide and maintain the necessary minimum gap distances (typically less than 1 mm) when forming a moving electrode type mechanism (for example, the parallel disk clutch configuration shown in FIG. 53(b). It is particularly difficult to consistently manufacture rotating electrode devices that maintain such a small gap distance. Thus, the prior art has failed to provide a design which is adaptable to mass production manufacturing techniques, contributing to the fact that, to date, there has been little or no commercial realization of the ER fluid effect.
The design of devices utilizing the ER fluid effect has also been hampered due to a variety of other problems. For example, ER fluids historically had a low yield strength and temperature sensitivity which severely limited their use in most applications. ER fluids also typically cannot tolerate the presence of common impurities, such as water or other contaminates, further limiting their applicability. Furthermore, the lack of available inexpensive, variable high-voltage power supplies limits the commercial feasibility of controllable ER fluid systems. Recently, a new generation of ER fluids has been developed which overcome many of the problems associated with low yield strength, temperature sensitivity, and the problems associated with the presence of water. Examples of ER fluids having a variety of advantageous properties and formulas are described, for example, in U.S. Pat. No. 3,970,573 issued to Westhaver; U.S. Pat. No. 4,129,513 issued to Stangroom; U.S. Pat. No. 4,483,788 issued to Stangroom et al.; U.S. Pat. No. 4,502,973 issued to Stangroom; U.S. Pat. No. 4,645,614 issued to Goossens; U.S. Pat. No. 4,812,251 issued to Stangroom; U.S. Pat. No. 4,744,914 issued to Filisko et al.; U.S. Pat. No. 4,772,407 issued to Carlson; U.S. Pat. No. 4,879,056 issued to Filisko; and U.S. Pat. No. 4,994,918 issued to Chung. These patent references show that there has been a great deal of advancement in the formulation of ER fluids. However, the prior art literature and the marketplace are still lacking in any commercially realizable application of the ER fluid phenomenon.
Another type of variable viscosity material which exhibits a change in flow characteristics in response to an applied field is known as a magneto-rheological fluid (MR fluid). MR fluids manifest a magneto-rheological effect in response to an applied magnetic field and are the true magnetic analogs of electro-rheological fluids (ER fluids). Typically, magneto-rheological fluids consist of micron-sized, magnetically polarizable particles dispersed in a carrier medium. The formation of particle chains (fibrils) upon the application of a magnetic field is the basic operation of a magneto-rheological fluid. In the presence of a shear force, the equilibrium that is established between the formation and braking of fibrils corresponds to the yield strength defined for the fluid. MR fluids exhibit Bingham plastic behavior similar to that of ER fluids. However, the yield stress values generated by MR fluids are significantly greater than those measured for their ER fluid counterparts. Currently, a good ER fluid will exhibit a yield stress greater than 3 kPa with an applied electrical field of 4 kV/mm. In contrast, yield stress values in excess of 80 kPa have been obtained for MR fluids upon the application of a magnetic field. Further, MR fluids have demonstrated the capability of operating over a temperature range of -40.degree. to 150.degree. C. with only a small variation in the yield strength of the MR fluid. MR fluids can respond to the application of a magnetic field on the milli-second time scale, similar to that exhibited by ER fluids. Furthermore, MR fluids are not affected by the presence of chemical impurities that normally are encountered in various processes used for mass manufacturing and can utilize raw materials that are non-toxic, environmentally safe, and compatible with most device components, while requiring a low voltage power supply (see, K. D. Weiss, et al., "High Strength Magneto-and Electro-Rheological Fluids" SAE Technical Papers Series #932451). Examples of magneto-rheological fluids are described in U.S. Pat. No. 4,992,190 issued to Shtarkman; and U.S. Pat. No. 5,167,850 issued to Shtarkman.
Another type of magnetically reactive fluid is known as ferrofluids. Ferrofluids or magnetic liquids are not the magnetic analogs of ER fluids. Ferrofluids typically consist of colloidal magnetic particles, such as magnetite and magnesium-zinc ferrite dispersed in a continuous carrier phase. Upon the application of a magnetic fluids, colloidal magnetic fields retain their liquid properties. Due to the effect of Brownian motion on the polarized particles, ferrofluids do not exhibit the ability to form particle fibrils or develop yield stress. Rather, these magnetic liquids experience a body force on the entire fluid which is proportional to the magnetic field gradient. This force causes these liquids to be attracted to regions of high magnetic field strength. An example of a ferrofluid is described in U.S. Pat. No. 4,687,596 issued to Borduz et al.
Fine, dry, stainless-steel powders have been utilized instead of friction pads to control loads in prior art magnetic particle clutches and brakes. An example of a prior art magnetic particle brake is shown in FIG. 53(c). A disk-shaft assembly is centered in a gap within a housing. The gap between the disk and the housing is filled with a fine, dry, stainless-steel powder. The powder is free-flowing until a magnetic field is applied from a stationary coil. When power is applied, the powder particles form chains along the magnetic field lines, linking the disk to the housing. A torque proportional to the magnetic field and to the applied input current is thus developed. These conventional magnetic particle clutches and brakes suffer from many of the same disadvantages as the prior art ER fluid or MR fluid clutches and brakes. In particular, the materials used and the manufacturing techniques utilized in forming the disk-shaft assembly and the housing, results in high production costs and non-adaptability to high-volume mass production manufacturing techniques. Typically, the manufacture of these devices requires expensive and labor consuming machining manufacturing processes, thereby, preventing the utilization of inexpensive mass production manufacturing techniques, such as injection molding, metal stamping, and electroplating.
Furthermore, all of the prior art devices utilize a smooth and flat rotating surface and depend upon the ability of the variable viscosity material to adhere to the smooth and flat rotating surface in order to develop resistance to rotation. Additionally, these prior art devices lack any means for easily maintaining a consistent gap distance between the rotating electrode member and the facing components (inner walls of the housing) of the clutch or brake.
A typical vehicle suspension system utilizes a conventional shock absorber as a force dampener. In a conventional shock absorber, a piston is attached to the vehicle body and moves through a fluid contained in a cylinder. The force required to cause the fluid to flow from one side of the piston to the other is approximately proportional to the velocity of the piston in the cylinder. The dampening force of the conventional shock absorber is controlled by the viscosity of the fluid and the size of an orifice in the piston through which the fluid flows. The orifice can pass only so much fluid in a given length of time. If the vehicle body to which the piston is attached is displaced faster than the liquid can transfer through the orifice, a bottoming effect occurs. This effect is experienced by the vehicle occupants when a pot hole in the street is hit at too fast a rate. The suspension springs may appear to bottom out, but actually, it is the conventional shock absorber which has bottomed out.
ER fluids have been utilized in prior art controllable force dampers. FIGS. 53(d) and 53(e) show example configurations of two prior art devices known as fixed plate and sliding plate dampers, respectively. In the fixed plate dampener, the dampening force on a piston is realized by the control of a pressure drop across valve-like channels through which the ER fluid is forced to flow. In a sliding plate dampener, the dampening force originates from the controlled shear resistance between the moving piston, which acts as one electrode, and adjacent parallel surfaces, which remain motionless and act as the other electrode. These prior art controllable force dampers have been typically ineffective for use in consistently controlling an appreciable force, and thus their commercial utilization has been extremely slight, if not non-existent. In the case of the parallel plate type prior art controllable dampener, an effective ER fluid valve has not been provided. Such an ER fluid valve should be easy to manufacture and be effective to control the flow rate of the ER fluid through it to such an extent as to be able to provide a commercially usable controllable force dampener. In the case of the sliding plate dampener, there is too little electrode surface area to develop the resistance due to the ER fluid effect necessary for such a device to be commercially acceptable.